Voltage booster for memory devices

Abstract

 The device (2) presents a first line (20) at the supply voltage (V_DD); a second line (21) at the boost voltage (V_B); a booster stage (10); a supply detecting stage (15) connected to the first line and generating a first level signal (VL) when the supply voltage exceeds a first predetermined level; a boost detecting stage (16) connected to the second line and generating a second level signal (BDET) when the boost voltage exceeds a second predetermined level; a regulating stage (12) enabled by the boost detecting stage (16); and a pump control stage (14), which generates a regulating enabling signal (RUP) for the regulating stage (12) in the absence of the first level signal and in the presence of an enabling signal (ENB) enabling the boost condition. The regulating stage (12) generates a regulating signal (REG) in the presence of the second level signal and the regulating enabling signal (RUP), when the boost voltage (V_B) exceeds a third predetermined level; and the pump control stage (14) generates a pump activating signal (POK) for the booster stage in the absence of the first level signal (VL) and the regulating signal (REG).

Description

[0001] The present invention relates to a voltage booster for memory devices.

[0002] As is known, memory devices operating at such a low voltage as not to permit direct biasing of the word lines to read the selected cells are provided with voltage boosters for generating a voltage higher than the supply voltage.

[0003] Known booster devices are invariably rigid solutions incapable of adapting to different operating conditions (e.g. different supply voltages), and either present high consumption or a slow response in switching to different operating modes (typically, from standby).

[0004] It is an object of the present invention to provide a voltage booster device which may be enabled and configured flexibly, is only turned on when supply voltage is low, is turned off automatically when the low supply voltage condition ceases, presents zero consumption when disabled, and is capable of controlling work conditions with no delay, by being turned on immediately after the memory device and before the first reading is performed.

[0005] According to the present invention, there is provided a voltage booster device for memory devices, as claimed in Claim 1.

[0006] A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows an overall diagram of a memory device comprising the booster device according to the invention;

Figures 2 to 4 show electric diagrams of parts of the booster device according to the present invention;

Figures 5 and 6 show time plots of a number of electric quantities in the Figure 1 diagram.

[0007] Number 1 in Figure 1 indicates a memory device comprising, for example, a nonvolatile memory such as an EPROM, of which only the parts pertinent to the present invention are shown.

[0008] Memory 1 comprises a voltage booster device 2; a memory array 3 including a number of memory cells 4; a logic stage 5 incorporating all the memory timing and control components not involved in controlling the boost functions; and a supply stage 6.

[0009] Device 2 in turn comprises a main pump 10; a standby pump 11; a boost regulating stage 12; a boost control stage 13; a pump control stage 14; a supply detecting stage 15; a boost detecting stage 16; a switching circuit 17; and a nonvolatile memory element 18, e.g. comprising an UPROM memory cell.

[0010] More specifically, and in known manner, supply stage 6 generates a supply voltage V_DD which is supplied, in a manner not shown, over line 20 to all the other stages of memory 1. Logic stage 5 generates a signal CE for enabling memory 1; a power-on reset signal POR for switching all the memory circuits to a known predetermined condition when the memory is turned on; a signal RR indicating memory array 3 is being read; and a signal BU indicating memory array 3 is being programmed; and is in turn supplied by boost control stage 13 with a signal BOK confirming a sufficient (supply or boost) voltage level to read cells 4.

[0011] Main pump 10 receives an enabling signal ENP and a main pump activating signal POK from pump control stage 14; standby pump 11 receives enabling signal ENP and a standby pump activating signal SP from pump control stage 14; and both pumps 10 and 11 are connected to a boost line 21 at voltage V_B. Boost regulating stage 12 receives a signal BDET (indicating the predetermined boost level has been reached) from boost detecting stage 16, signals RUP, RDW (for enabling and disabling regulation of the boost line) from pump control stage 14, and itself generates an active regulating signal REG. Boost control stage 13 receives a signal VL (low supply voltage) and a signal STE (for disabling the autoboost procedure enabled automatically when the memory is turned on) from supply detecting stage 15, signal BDET from boost detecting stage 16, signals CE, BU and POR from logic stage 5, a signal SBF from memory element 18, and itself generates signals BOK, SETB (boost enable) and ENB (which is enabled following initial power-on reset and when the memory is active, and is disabled during standby or in the event of forced disabling of the booster. Pump control stage 14 receives signals RR, BU, POR, VL, SETB, REG, ENB, and generates signals RUP, RDW, BM (indicating boost operating mode), VM (indicating high-voltage operating mode) and SBB (for memorizing the operating or non-operating condition of the booster). Supply detecting stage 15 receives signals REG, ENB and supply voltage V_DD, and generates signals VL and STE. Boost detecting stage 16 receives signal RUP, generates signal BDET, and is connected to boost line 21. Switching circuit 17 receives signals BM, VM, SBB, and is connected to boost line 21; and nonvolatile memory element 18 generates signal SBF.

[0012] When the Figure 1 memory is turned on, the high POR signal resets the circuits (signals SETB, ENB, BOK, ENP, POK, SP, BDET, RUP, SBB, VM low, and signals VL, REG, RDW, STE, BM high) to set the supply operating mode, so that switching circuit 17 connects boost line 21 to supply line 20 and the pumps are turned off. When the POR signal switches to low after turn-on, and in the absence of disabling conditions (by SBF), SETB switches to set the booster, and signal ENB switches to enable the autoboost condition. At the same time, pump control stage 14 is set to boost mode, turns on both pumps 10, 11 via signals ENP, POK, SP, and controls switching circuit 17 via signals VM, BM, SBB until it disconnects boost line 21 from supply line 20. The switching of signal ENB also enables detecting stages 15, 16.

[0013] As supply voltage V_DD increases, while pumps 10, 11 are still turned on, various situations arise, depending on whether memory 1 operates at low (3 V) or high (5 V) voltage, and depending on the speed with which the supply voltage increases.

[0014] If memory 1 operates at high voltage, V_DD, as it increases, eventually reaches the predetermined threshold value of supply detecting stage 15, which switches VL to low, thus switching pump control stage 14, which turns off the pumps by switching signals ENP, POK, SP to low, controls switching circuit 17 so that it connects boost line 21 to supply line 20, memorizes the present operating condition (SBB low), and disables boost detecting stage 16 and boost regulating stage 12 via low signal RUP. Also, boost control stage 13 switches signal BOK to indicate a sufficient read supply has been reached, and supply detecting stage 15 disables the autoboost condition by switching signal STE to low.

[0015] Conversely, if the memory operates at low voltage, signal VL is not switched, and boost line 21, pulled up by pumps 10, 11 which have not been turned off, eventually exceeds the predetermined value of boost detecting stage 16, which therefore switches signal BDET to high. Boost regulating stage 12, already enabled via signal RUP by pump control stage 14 (still in boost mode), is therefore enabled and commences its regulating function, which is scanned by the switching sequence of signal REG between high and low. The first high-to-low switch of signal REG provides, with a given delay, for disabling supply detecting stage 15, which keeps signal VL high and, as previously, switches signal STE to disable the autoboost condition. And each time signal REG switches to low, it switches signals SP and POK to low to temporarily turn off the pumps, but with no change in the boost operating mode condition of the pump control stage, so that, when voltage V_B on boost line 21 falls below the predetermined threshold, low-to-high switching of signal REG turns the pumps back on.

[0016] In the event memory 1 operates at high voltage, but supply voltage V_DD is slow to increase, boost voltage V_B may exceed its threshold value so that boost detecting stage 16 is triggered before supply detecting stage 15. In view of the delay, however, between the instant in which boost detecting stage 16 is triggered and signal REG switches the first time, and between the instant in which the trailing edge of signal REG is received and supply detecting stage 15 is turned off, supply line 20 manages to reach its predetermined threshold value before detecting stage 15 is turned off, so that signal VL also switches, the device switches to high-voltage operating mode, boost regulating stage 12 is turned off, and pump control stage 14 switches as described above.

[0017] When memory 1 switches to standby, logic stage 5 switches signal CE to low, and boost control stage 13, no longer sustained by the autoboost situation disabled previously, switches signal ENB to low. Depending on the operating situation prior to the memory switching to standby and memorized in pump control stage 14, this provides for keeping both pumps 10, 11 off, if the memory was operating in high-voltage mode, and for keeping standby pump 11 on and turning off main pump 10, if the memory was operating in boost mode.

[0018] When the memory switches from standby, boost control stage 13 switches signal ENB to high to reactivate the supply voltage V_DD and boost voltage V_B detecting functions of stages 15 and 16; pump control stage 14 adapts to the existing condition, in the same way as after power-on; and regulation is at least temporarily blocked by the high state of signal BDET, until the device reaches the required operating condition, in the same way as described above relative to when the POR signal switches to low, except that in this case the autoboost procedure remains disabled.

[0019] When signal BU switches to low during programming, pump control stage 13 disables pumps 10, 11 via signals ENP, POK and SP; and when signal BU switches back to high at the end of programming, the whole of memory device 1 is reactivated, thus reactivating the autoboost procedure and detection of supply and boost voltages V_DD and V_B, as by the POR signal when the device is turned on.

[0020] The structure and operation of the various stages of device 1 will now be described in detail.

[0021] As stated, boost regulating stage 12 provides for generating regulating signal REG, which, depending on its logic stage, enables or disables pumps 10, 11 to maintain an optimum level of boost voltage V_B. Stage 12, the structure of which is not shown in detail, substantially comprises a threshold element, which compares the boost voltage V_B on boost line 21 with a predetermined threshold value, generates a high output signal REG as long as V_B is below the threshold, and switches signal REG to low as soon as V_B exceeds the threshold. Stage 12 is enabled by signal RUP and disabled by signal RDW; and high-to-low switching of output signal REG is only permitted after signal BDET switches to low, i.e. indicating the boost line has reached the minimum operating level determined by boost detecting stage 16. An advantageous embodiment of a regulating stage capable of setting an optimum boost level for reading the cells as a function of their conductivity, and using an actual memory cell as the threshold element to appropriately simulate the condition of memory cells 3, is described, for example, in European Patent Application n. 96830026.9 filed on 24.1.96 by the present Applicant.

[0022] Boost control stage 13 is shown in detail in Figure 2, and substantially comprises an autoboost circuit 25; a boost preset circuit 26; a memory read enabling circuit 27; and a boost enabling circuit 28. More specifically, autoboost circuit 25 is formed by a flip-flop, and comprises two NAND gates 30, 31; NAND gate 30 presents a first input receiving signal STE, a second input connected to the output of NAND gate 31, and an output connected to a first input of NAND gate 31; and NAND gate 31 presents a second input connected to a node 32 and receiving signal SETB, and an output defining a node 33 presenting a signal AB. Boost preset circuit 26 comprises an inverter 34 receiving signal BU; and a three-input NOR gate 35, one input of which is connected to the output of inverter 34, the other inputs of which receive signals POR and SBF, and the output of which defines node 32. Memory read enabling circuit 27 comprises a NAND gate 36 having one input connected to node 32, another input receiving signal VL, and an output connected to one input of an AND gate 37, which presents another input connected to a node 38 presenting an inverted delayed-power-on signal DPn; the output of AND gate 37 is connected to one input of a NOR gate 39, another input of which receives signal BDET, and the output of which is connected to an inverter 40, the output (node 41) of which presents signal BOK; and node 38 is connected to an input 42 of circuit 27, which supplies the power-on-reset POR signal via a cascade of inverters 44 and capacitors 45 forming a delaying and inverting branch, so that signal DPn is delayed and inverted in relation to the POR signal. Boost enabling circuit 28 comprises an OR gate 47 having a first input connected to node 33, a second input receiving signal CE, and an output connected to one input of a NAND gate 48, another input of which is connected to node 32, and the output of which is connected to an inverter 49 generating signal ENB at output node 50.

[0023] Boost control stage 13 in Figure 2 operates as follows. When memory 1 is turned on, the POR signal is high; signal BDET is low; and signals STE, BU and VL are high. Assuming signal SBF is low (memory element set and the boost condition enabled), then the output of NAND gate 35 and signal SETB are low; the output of NAND gate 48 is high and signal ENB low; all boost activity is therefore disabled as explained below; and the low SETB signal sets flip-flop 25 so that signal AB is high to preset the autoboost condition after power-on, and produces a high output of NAND gate 36. On the other hand, DPn is low, the output of AND gate 37 is low, and signal BOK is low indicating an unstable condition unsuitable for reading the cells.

[0024] The same boost disabling condition (signals SETB and ENB low) also applies when memory element 18 is not set (signal SBF high) and when programming the memory array (signal BU low).

[0025] The high STE signal, together with the reset condition produced by the high AB signal, maintains a low logic level at the input of NAND gate 31 not connected to node 32, so as to enable autoboost circuit 25. Consequently, when the POR signal switches to low after turn-on, and signal SETB switches to high, flip-flop 25 nevertheless maintains a high AB signal and a high output of OR gate 47 despite signal CE being low; NAND gate 48, receiving two "1", switches to low; and signal ENB at output 50 switches to high. At this phase, in which supply voltage V_DD is increasing to its steady-state value, VL is definitely still high; the output of NAND gate 36 switches to low; and signal DPn, which switches with a given delay in relation to the POR signal, is still low, so that the output of AND gate 37 is low, BDET is still low, and signal BOK remains low. At this phase, the delay in the switching of DPn in relation to the POR signal prevents gate 37 (and hence signal BOK) from switching in the brief interval between the switching of the POR signal and signal SETB.

[0026] Upon supply voltage V_DD reaching a high operating value (VL switches to low), gate 36 switches to high and, as soon as DPn switches to "1", switches signal BOK to high to indicate the read condition has been reached. Similarly, when BDET switches to high, signal BOK also switches to high.

[0027] When high, signal CE in stage 13 maintains a high output of gate 47 regardless of the condition of autoboost flip-flop 25, so that signal ENB remains high and the boost condition enabled. Conversely, if CE is low, signal ENB is high as long as signal AB generated by autoboost flip-flop 25 remains high, and switches to low (to disable the boost condition) when AB switches to low.

[0028] The above enabling procedure following turn-on of memory 1 also applies at the end of programming, when signal BU switches back to high.

[0029] When VL switches, or when regulation of pumps 10, 11 is established, signal STE switches to low with a given delay, as will be seen with reference to Figure 3, so as to switch the output of NAND gate 30 to high and the output of NAND gate 31 (signal AB) to low. Once the booster device is stabilized, the autoboost condition is disabled and can only be reactivated when memory 1 is initialized (by the POR signal) or programmed, as described above. Consequently, when memory 1 switches to standby (signal CE supplied by logic stage 5 switches to low), signal ENB also switches to low, even if SETB remains high.

[0030] The basic structure of supply detecting stage 15 is shown in Figure 3. Stage 15, which is described and illustrated in detail with reference to Figure 6 in European Patent Application n. 95830360.4 of 4.8.95, comprises a threshold comparator 53 (e.g. employing a memory cell as in boost regulating stage 12) connected to supply line 20 at V_DD, and having an enabling input 54, a disabling input 55, and an output 56 generating signal VL, the high value of which indicates a low supply voltage (e.g. below 4 V). Input 54 is connected to an input 57 of stage 15, which supplies signal ENB via a switch 58 having a pair of complementary control terminals; and input 55 is connected to the same input 57 of stage 15 via an inverter 59.

[0031] Stage 15 also comprises a disabling network 60 for disabling stage 15 when supply voltage V_DD reaches the set threshold value, or when the regulating function is activated. Network 60 substantially comprises a flip-flop 61, and an inverting delay block 62; flip-flop 61 comprises a first and second NAND gate 63, 64; NAND gate 63 has a first input connected to the output 56 of threshold comparator 53, a second input connected to the output of NAND gate 64, and an output 68 connected to inverting delay block 62; and NAND gate 64 has a first input connected to the output of gate 63, a second input connected to the output 56 of threshold comparator 53, and a third input receiving signal REG.

[0032] The output of inverting delay block 62 is connected to the power-off terminal of switch 58 and to the input of an inverter 65, the output 66 of which is connected to the power-on terminal of switch 58 and supplies signal STE.

[0033] Supply detecting stage 15 operates as follows. To begin with, when ENB is low, the output 68 of gate 63 is high, the output of inverting block 62 is low, and STE is high; switch 58 is therefore turned on; threshold comparator 53 receives the low ENB signal at set input 54, a high signal at reset input 55, and is therefore reset; VL is high; and NAND gate 64, receiving three high input signals, supplies NAND gate 63 with a low signal to confirm the present state.

[0034] When ENB switches to high, threshold comparator 53, receiving a high signal at set input 54 and a low signal at reset input 55, is enabled. VL, however, remains high as long as supply voltage V_DD remains below the predetermined threshold, so that NAND gate 64 does not switch, and maintains a "0" at the input of gate 63, so that disabling network 60 does not switch. Network 60 only switches when supply voltage V_DD reaches the predetermined high threshold value (VL switches to low) or when the memory switches to regulating mode (REG switches to low). In both cases, the presence of a "0" at the input of NAND gate 64 switches output 68 of flip-flop 61 to low. After a delay determined by block 62, the output of block 62 itself switches to high, thus switching STE to low and opening switch 58; input 54 of threshold comparator 53 no longer receives the enabling signal and is therefore disabled; but, since the signal at input 55 is still low, comparator 53 does not alter the former level of signal VL; and any changes to signal REG have no effect on the present condition, unless a standby or programming condition switches ENB to low, thus resetting stage 15.

[0035] Boost detecting stage 16, an advantageous embodiment of which is illustrated and described with reference to Figure 3 in European Patent Application n. 95830363.4 of 4.8.95, substantially comprises a threshold element (preferably using a memory cell) connected to boost line 21 and enabled by signal RUP. When signal RUP is high, detecting stage 16 monitors boost voltage V_B and generates signal BDET, which is low as long as boost voltage V_B remains below a predetermined threshold, and switches to high when the threshold is exceeded. Signal BDET is also low when detecting stage 16 is disabled.

[0036] The structure of pump control stage 14 and switching circuit 17 is shown in detail in Figure 4. More specifically, pump control stage 14 comprises a control circuit 70 and a standby set circuit 71.

[0037] Standby set circuit 71 comprises a flip-flop 73 comprising two PMOS transistors 74, 75 and two NMOS transistors 76, 77. PMOS transistors 74, 75 have the source terminals connected to supply line 20 at V_DD, the gate terminals connected to respective nodes 79, 80, and the drain terminals connected to respective nodes 80, 79; and transistor 75 is a native type with a high threshold. NMOS transistors 76, 77 have the source terminals grounded, the gate terminals connected to respective nodes 79, 80, and the drain terminals connected to respective nodes 80, 79.

[0038] Node 79 is grounded via two NMOS transistors 81, 82 in series, the gate terminals of which respectively receive signal ENB from boost control stage 13, and signal BM generated by control circuit 70; and is also connected to the input of an inverter 87, the output (node 88) of which supplies signal SBB. Node 80 is grounded via two NMOS transistors 84, 85 in series, the gate terminals of which respectively receive signal ENB, and signal VM generated by control circuit 70; and is also grounded via an NMOS transistor 86, the control terminal of which receives the power-on-reset POR signal from stage 5.

[0039] Control circuit 70 comprises a NAND gate 90, the inputs of which receive signals VL and ENB, and the output of which generates signal RDW; an inverter 91 connected to the output of NAND gate 90; and a NOR gate 92, one input of which receives a signal SBBn (inverted in relation to SBB) generated by switching circuit 17, and the other input of which receives signal ENB. The output of inverter 91 (presenting signal RUP) and the output of NOR gate 92 are connected to the inputs of an OR gate 93, the output of which is connected to one input of a NAND gate 94, the other input of which receives signal SETB from stage 13. The output of NAND gate 94 defines a node 95 presenting signal VM, and is connected to an inverter 96, the output of which defines a node 97 presenting signal BM. Node 97 is connected to one input of a three-input NAND gate 98, which also receives signals REG and BU; and the output of NAND gate 98 is connected to an inverter 99, the output (node 100) of which supplies signal SP to standby pump 11.

[0040] Control circuit 70 also comprises a NOR gate 101 having a first input connected to the output of NAND gate 90, a second input connected to the output of NAND gate 98, a third input receiving signal RR from stage 5, and an output 104 supplying signal POK; and an AND gate 102 having two inputs supplied with signals SETB and VL, and an output 103 supplying signal ENP to standby pump 11.

[0041] Switching circuit 17 comprises a first and second latch 105, 106 connected between boost line 21 and ground and respectively controlling a transistor 107, and a pair of push-pull transistors 108, 109 connecting the well region of transistor 107 to the highest voltage point each time. More specifically, latch 105 comprises two PMOS transistors 110, 111 and two NMOS transistors 112, 113. PMOS transistors 110, 111 have the source terminals and well regions connected to boost line 21 at V_B, the gate terminals connected to respective nodes 114, 115, and the drain terminals connected to respective nodes 115, 114; and NMOS transistors 112, 113 have the source terminals grounded, the gate terminals connected to respective nodes 97, 95 of control circuit 70, and the drain terminals connected to respective nodes 115, 114.

[0042] Node 114 is connected to the gate terminal of PMOS transistor 107, which has the source terminal connected to supply line 20 at V_DD, the drain terminal connected to boost line 21 at V_B, and the well region connected to a node 117. Latch 106 comprises two PMOS transistors 120, 121, and two NMOS transistors 122, 123. More specifically, PMOS transistors 120, 121 have the source terminals and well regions connected to boost line 21, the gate terminals connected to respective nodes 124, 125, and the drain terminals connected to respective nodes 125, 124; and NMOS transistors 122, 123 have the source terminals grounded, the gate terminals connected respectively to node 88 of boost preset circuit 71 and to the output 127 of an inverter 128 (generating signal SBBn supplied to control circuit 70), and the drain terminals connected to respective nodes 125, 124.

[0043] PMOS transistor 109 has the source terminal connected to supply line 20, the gate terminal connected to node 124 of latch 106, and the drain terminal and well region connected to node 117; and PMOS transistor 108 has the source terminal connected to boost line 21, the gate terminal connected to node 127, and the drain terminal and well region connected to node 117.

[0044] The Figure 4 circuit operates as follows. When memory 1 is turned on, the high POR signal keeps node 80 low, node 88 high, and node 88 (signal SBB) low; and, as stated, ENB and SETB are low, and VL high. The low SETB signal produces a high output of NAND gate 94, so that VM is high, BM is low, SP and POK are low, and signal ENP at output 103 of AND gate 102 is low, so that pumps 10 and 11 are turned off. As VM is high and BM low, transistor 113 of switching circuit 17 is turned on, node 114 is low, and transistor 107 is turned on to connect boost line 21 to supply line 20, so that, at turn-on, boost voltage V_B follows the increase in supply voltage V_DD exactly, with substantially no losses. As SBB is low, transistor 122 is turned off, transistor 123 is turned on, node 124 is low, and node 125 is high, so that transistor 109 is turned on and connects the well region of transistor 107 (node 117) to supply line 20, transistor 108 is turned off, and signal SBBn is high.

[0045] The above condition at power-on with the POR signal enabled (pumps disabled, boost line 21 connected to supply line 20) also applies when memory element 18 (Figure 1) is not set (SBF high) and when memory 1 is being programmed (BU high), in that both conditions determine a low SETB signal.

[0046] When signal SETB switches to high at the end of the power-on phase, it enables gates 94 and 102, and no longer has any effect on the operation of pump control stage 14 or switching circuit 17, unless the boost condition is disabled by SBF or BU.

[0047] Switching of signal ENB to high (with signals VL, REG, BU stable in the high state, and signal RR stable in the low state) switches gates 90, 91, 93, 94; signal ENP at the output of gate 102 switches to high to enable pumps 10, 11; the device switches to boost mode with VM low and BM high, so that gates 98, 99, 101 switch, and signals SP and POK for respectively activating standby pump 11 and main pump 10 switch to high to activate both pumps. The switching of signals VM and BM also switches latch 105, node 114 of which switches to high to turn off transistor 107; boost line 21 is disconnected from supply line 20; and flip-flop 73 also switches to switch signal SBB to high, memorize activation of pumps 10, 11, and switch latch 106. More specifically, transistor 122 is turned on; transistor 123 is turned off; node 124 switches to high to turn off transistor 109; node 127 (signal SBBn) switches to low to turn on transistor 108; the well region of transistor 107 (node 117) is connected to boost line 21, now higher than supply line 20; and the low switching of SBBn has no effect on the state of NOR gate 92, in view of the high value of signal ENB.

[0048] If, as it increases, supply voltage V_DD reaches the predetermined threshold value of supply detecting stage 15, and signal VL switches to low, as stated, NAND gate 90 of control circuit 70 switches, signal RDW switches to high, and signal RUP switches to low, thus disabling boost regulating stage 12 and boost detecting stage 16 as explained above. Moreover, since the output of NOR gate 92 was also low (ENB = 1), the output of OR gate 93 switches to low, signal VM again switches to high, and signal BM switches to low to return to supply voltage mode. Consequently, flip-flop 73 switches to the initial state with signal SBB low; latch 105 switches so that node 114 switches to low; transistor 107 is turned back on to reconnect boost line 21 to supply line 20; latch 106 switches; node 127 (signal SBBn) switches to high; transistor 108 is turned off; transistor 109 is turned on to reconnect the well region of transistor 107 to supply line 20; and signals SP, POK and ENP switch to low to disable both pumps 10, 11.

[0049] Conversely, when memory 1 operates at low voltage and signal VL remains high, pump control stage 14, even after BDET switches, remains in the boost condition with signal VM low, signal BM high, signal SBB high, signal SBBn low, and with boost line 21 disconnected from supply line 20. When boost regulating stage 12 is activated and output signal REG commences the switching sequence, pumps 10, 11 are so controlled by signals SP and POK as to be turned off and on when boost line 21 respectively exceeds and is below the threshold of boost regulating stage 12.

[0050] If signal BU switches to low in boost mode (during programming), it switches signals POK and SP to low to turn off both pumps and also reset the whole of pump control stage 14 due to the low SETB signal. Conversely, low-to-high switching of signal RR when reading the memory array only switches signal POK to low, so that only main pump 10 is turned off, and standby pump 11 continues operating to prevent a sharp variation, and hence noise, on boost line 21 when evaluating the data.

[0051] When memory 1 switches to standby and signal CE switches to low, as stated, boost control stage 13 switches signal ENB to low, so that flip-flop 73 is frozen in the state memorized previously by transistors 81, 84 being turned off and cutting off the connection between standby set circuit 71 and control circuit 70. The switching of signal ENB also disables NAND gate 90, the output (signal RWD) of which switches to high, regardless of signal VL, to disable boost regulating stage 12.

[0052] If, before signal ENB switched, signal SBB was low and signal SBBn high (high supply voltage, pumps 10, 11 turned off), the output of NOR gate 92 remains low; since signal RUP is low, the output of OR gate 93 remains low, and the output of NAND gate 94 is high, so that VM is high and BM low; signal SP is therefore maintained low; and signal POK is maintained low by the high RDW signal.

[0053] Conversely, if signal SBBn was low (boost condition) before signal ENB switched, the output of gate 92 and the output of OR gate 93 switch to high, so that signal VM remains low and signal BM high. And since, as stated, signals REG, VL and BU are high, signals SP and ENP are maintained high, whereas signal POK switches to low on account of the high RDW signal, so that only standby pump 11 is operated and main pump 10 is disabled.

[0054] In standby mode, switching circuit 17 remains in the previously set state, i.e. with transistor 107 turned off and boost line 21 isolated from supply line 20, in the case of boost mode, and with transistor 107 turned on and boost line 21 connected to supply line 20 in the case of high-voltage mode.

[0055] When signal ENB switches back to high at the end of standby, stages 14, 15 and 16 are reset, supply voltage V_DD and boost voltage V_B are again monitored, and the pumps are again controlled by control stage 14.

[0056] Pumps 10, 11 may be formed in any known manner. For example, advantageous solutions as regards the main pump and the standby pump are described respectively in European Patent Application n. 95830335.6 of 28.7.95, and in copending European Patent Application entitled "Standby voltage booster stage for a memory device", both filed by the present Applicant. By forming the pumps so that the main pump is capable of bringing the boost voltage to 3V_DD-2V_T (where V_T is the gate-source voltage drop of the switches for discharging the capacitors) and the standby pump is capable of bringing the boost voltage to 2V_DD-V_T, a more efficient pumping effect is achieved at the initial boost phase by first exploiting the greater initial speed of standby pump 11 and then the greater capacity of main pump 10, as shown in the Figure 5 plot, in which curve A indicates pull-up of boost line 21 by standby pump 11 only, curve B by main pump 10 only, and curve C by the combined effect of both pumps 10, 11 as described above.

[0057] Figure 6 shows a graph of supply voltage V_DD and boost voltage V_B during turn-on and immediately following disabling of the POR signal. In particular, curves 1 and 2 show V_DD and V_B in the case of a low supply voltage (3 V), and curve 3 shows V_DD in the case of a high supply voltage (5 V) increasing more slowly than V_B. The plot also shows the POR signal and the trigger level of supply and boost detecting stages 15, 16.

[0058] The advantages of the device described are as follows. In particular, it is enabled and configured flexibly by programming a memory element; it automatically sets the operating mode (with or without the boost function) according to a predetermined low supply voltage; it presents no consumption in high-voltage operating mode or when the supply line is stabilized, and very little consumption in boost operating mode; and it provides for altering the operating mode setting alongside changes in previous supply conditions by automatically disabling the boost function when the low supply voltage condition ceases.

[0059] By virtue of supply line 20 and boost line 21 being connected directly when the device is turned on (POR signal enabled), boost line 21, at the start of the next autoboost phase, is pulled up by pumps 10, 11 as of the maximum available value (the V_DD value reached at that point) with no losses due to voltage drops across other components. Moreover, at the power-on phase, pumps 10, 11 are turned off and therefore in no way hinder pull-up of the boost line, while the efficiency of the next pumping phase is enhanced by the presence of both pumps, as stated above.

[0060] The supply detecting stage monitors the supply line each time the device is turned on and reset (after standby and programming), and is turned off automatically to avoid unnecessary consumption when a stable supply condition is reached, with a given delay to prevent minor transient states from cutting off the monitoring function before the device is stabilized.

[0061] To prevent read errors, the device only enables the read functions when appropriate voltage levels, monitored by detecting stages 15, 16, have been reached.

[0062] Clearly, changes may be made to the device as described and illustrated herein without, however, departing from the scope of the present invention.

Claims

1. A voltage booster device (2) for a memory device (1), characterized in that it comprises, in combination:

- a first line (20) at a first reference potential (V_DD);

- a second line (21) at a second reference potential (V_B);

- a first voltage booster stage (10) connected to said first and second lines;

- a supply detecting stage (15) connected to said first line, for generating a first level signal (VL) when said first reference potential exceeds a first predetermined level;

- a boost detecting stage (16) connected to said second line, for generating a second level signal (BDET) when said second reference potential exceeds a second predetermined level;

- a regulating stage (12) connected to said second line and having an enabling input connected to said boost detecting stage (16);

- a pump control stage (14) connected to said supply and boost detecting stages, to said regulating stage, and to said first voltage booster stage, for generating a regulating activating signal (RUP) for activating said regulating stage in the absence of said first level signal (VL); and

- a boost control stage (13) including boost self-activating means (25) receiving a power-on reset signal (POR), said first level signal (VL) and said second level signal (BDET), and for generating an activating signal (AB); and boost enabling means (28) for generating an enabling signal (ENB) for said pump control stage (14), said supply detecting stage (15) and said boost detecting stage (16) in the absence of said power-on reset signal (POR) and said first and second level signals (VL, BDET) and in the presence of said activating signal;
   said regulating stage (12) generating a regulating signal (REG) in the presence of said second level signal (BDET) and said regulating activating signal (RUP), and when said second reference potential (V_B) exceeds a third predetermined level;
   and said pump control stage (14) generating a first pump activating signal (POK) for said first booster stage (10) in the absence of said first level signal (VL) and said regulating signal (REG).

2. A device as claimed in Claim 1, characterized by a second booster stage (11) connected between said first and second lines (20, 21).

3. A device as claimed in Claim 1 or 2, characterized in that said boost enabling means (28) has an input receiving a standby signal (CE) and an input receiving said power-on reset signal (POR), and generates said enabling signal in the absence of said power-on reset and standby signals.

4. A device as claimed in Claim 3, characterized in that said boost control stage (13) also comprises boost disabling means (35) receiving a disabling signal (SBF, BU) for disabling said boost enabling means (28) in the presence of said disabling signal (SBF, BU).

5. A device as claimed in one of the foregoing Claims from 3 to 5, characterized in that said boost control stage (13) comprises read level indicating means (77) receiving said first and second level signals (VL, BDET) and said power-on reset signal (POR), and generating a read level signal (BOK) in the presence of said first or second level signal and in the absence of said power-on reset signal (POR).

6. A device as claimed in any one of the foregoing Claims from 3 to 5, characterized in that said supply detecting stage (15) comprises a first input (57) receiving said enabling signal (ENB); a second input receiving said regulating signal (REG); an output (56) supplying said first level signal (VL); a detecting circuit (53) connected to said first input and to said first line (20); and a self-disabling circuit (60) connected to said output (56) of said supply detecting stage (15) and to said second input, and generating a disabling signal (STE) disabling said supply detecting stage (15) on receiving said first level signal (VL) or said regulating signal (REG).

7. A device as claimed in any one of the foregoing Claims from 3 to 6, characterized in that said pump control stage (14) comprises a first input receiving said first level signal (VL); a second input receiving said enabling signal (ENB); and first logic means (90, 91, 94) connected to said first and second inputs of said pump control stage (14) and generating an operating mode signal (VM) having a first and second logic state respectively indicating a supply operating mode and a boost operating mode; said operating mode signal (VM) presenting said first logic state in the presence of said first level signal (VL) and said enabling signal (ENB), and said second logic state in the presence of said enabling signal and in the absence of said first level signal.

8. A device as claimed in Claim 7, characterized in that said first logic means (90, 91, 94) comprise initialization set means (94) receiving said power-on reset signal (POR) and determining said first logic state of said operating mode signal (VM) in the presence of said power-on reset signal (POR).

9. A device as claimed in Claim 8, characterized in that said pump control stage (14) comprises memory means (75) receiving said operating mode signal (VM) and generating a mode memory signal (SBB); and second logic means (93) connected to said memory means, to said second input of said pump control stage (14), and to said first logic means (90, 91, 94), and determining said first logic state of said operating mode signal (VM) in the absence of said enabling signal (ENB) and in the presence of a first value of said mode memory signal (SBB), and determining said second logic state in the absence of said enabling signal (ENB) and in the presence of a second value of said mode memory signal (SBB).

10. A device as claimed in Claim 9, characterized in that said pump control stage (14) comprises reset means (86) receiving said power-on reset signal (POR), connected to said memory means (75) and determining said first value of said mode memory signal (SBB) in the presence of said power-on reset signal (POR).

11. A device as claimed in Claims 2 and 8, characterized in that said pump control stage (14) comprises a first output (104) connected to said first voltage booster stage (10) and supplying said first pump activating signal (POK); a second output (100) connected to said second voltage booster stage (11) and supplying a second pump activating signal (SP); and third logic means (98, 101) connected to said first logic means (90, 91, 94), to said first input and said first and second outputs (104, 100) of said pump control stage (14), receiving said enabling (ENB) and operating mode (SBB) signals, generating said first and second pump activating signals (POK, SP) in the presence of said enabling signal (ENB) and said second logic state of said operating mode signal (VM), and generating said second pump activating signal (SP) in the absence of said enabling signal and said second logic state of said operating mode signal (SBB).

12. A device as claimed in Claim 11, characterized in that said third logic means (98, 101) comprises first disabling means (98) receiving said regulating signal (REG) and a programming signal (BU), and disabling said first and second pump activating signals (POK, SP) in the presence of said regulating and programming signals.

13. A device as claimed in one of the foregoing Claims from 8 to 12, characterized by a switching stage (17) interposed between said first and second lines (20, 21) and having a control input (95) receiving said operating mode signal (VM); said switching stage being closed and connecting said first and second lines in the presence of said first logic state of said operating mode signal (VM), and being open and disconnecting said first and second lines in the presence of said second logic state of said operating mode signal.

14. A device as claimed in Claim 13, characterized in that said switching stage (17) comprises a MOS transistor (107) having a first terminal connected to said first line (20), a second terminal connected to said second line (21), and a control terminal (114) connected to said control input.

Drawing